Environmental Engineering Reference
In-Depth Information
10.
1.
9.
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8.
7.
1. North American deserts
2. Atacama desert
3. Sahara desert
4. Namib desert
5. Kalahari desert
6. Arabian deserts
7. Thar desert
8. Taklimakan (Gobi) desert
9. Desert of the Loess Plateau
10. Central Asian deserts and steppe
11. Australian deserts
3.
6.
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11.
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FIGURE 1.1
Map showing world-wide distribution of the warm deserts.
latitudes, but, geomorphologically, they are very different from the subtropical deserts and
are excluded from this review.
1.2 The Sahara and Its Margins
The Sahara is the world's largest desert (covering c. 2.7 million miles 2 ), and the region
comprising the Sahara and the Nile occupies about half of the entire African continent. 4
The greater part of the region is free of surface water and is sparsely vegetated, and, being
exposed to dry, descending, northeasterly airstreams, its mean annual rainfall is less than
16 in. and over vast areas less than 4 in. 5 Temperatures are also high, and evaporation
losses from free water surfaces and transpiration losses from vegetated areas are greater
than anywhere else on the globe.*
The general morphology of the Sahara has been discussed by Mainguet, 7 who suggests
that its most distinctive characteristic, save only the relief provided by the Hoggar and
Tibesti massifs, is its flatness. This flatness is associated with great sandstone plateaus, a
series of broad, closed basins (of which Chad is the most notable), and a series of wind-
molded landscapes, which include deflational regs , corrasional fields of yardangs , and areas
of sand deposition ( ergs ). Some of the major geological, geomorphological, and climate
features of the area are shown in Figure 1.2.
Like most other deserts, the Sahara shows the imprint of a long evolution. For example,
the Sahara was glaciated in Ordovician times, when from palaeomagnetic evidence, it is
apparent that the South Pole was located about the center of this region. Well-preserved
* General discussions of the Saharan environments are provided by E. F. Gautier, J. L. Cloudsley-Thompson,
and R. Capot-Rey. 6
Regs are gravelly plains, typically formed by wind deflation. Yardangs are wind-sculpted landforms, and
corrasion is a process akin to sand blasting. Ergs are sand seas composed of a large area of dunes. Duricrusts
are exposed soil horizons cemented by various minerals and compounds, typically calcium carbonate
(calcretes), silica (silcretes), or iron compounds (fericretes).
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